1. Field of the Invention
The present invention relates to a film forming apparatus for manufacturing a structure by spraying a powder on a substrate at high speed so as to deposit the powder.
2. Description of a Related Art
Recent years, in the field of the micro electrical mechanical system (MEMS), fabrication of sensors, actuators, or the like employing piezoelectric ceramic by using film formation methods has been studied in order to further integrate those elements for practical use. As one of the film formation methods, the aerosol deposition method, which is known as a technology for forming a film of ceramic, metal, etc., receives attention. The aerosol deposition method (hereinafter, also referred to as “AD method”) is a method of generating an aerosol containing raw material powder and spraying it on the substrate to deposit the powder due to the collision energy at that time and form a film, which method is also referred to as spray deposition method or gas deposition method. Here, the aerosol is referred to as fine particles of a solid or liquid floating in a gas.
Japanese Patent Application Publication JP-P2001-348659A discloses an apparatus for fabricating a ceramic structure according to the aerosol deposition method. As shown in FIG. 1 of the document, in the aerosol deposition method, micro powder of the order of submicron is used as a raw material. The micro powder of the row material is provided within an aerosol generator 13, and a carrier gas such as nitrogen (N2) is ejected from a compressed gas cylinder 11 via a carrier pipe 2, and thereby, the raw material powder is blown up and floats in the carrier gas to generate the aerosol. On the other hand, the air inside of a structure forming chamber 14 is exhausted by an exhaust pump 18 and a substrate 16 held by a substrate holder 17 is provided therein. When the aerosol introduced from the aerosol generator 13 via the carrier pipe 12 is sprayed toward the substrate 16 from a nozzle 15, the raw material powder is accelerated by a high speed air flow, impinges on the substrate 16, and is deposited thereon.
However, in such an apparatus for fabricating a ceramic structure (film forming apparatus), there occurs a problem that the thickness of the structure formed on the substrate cannot be controlled accurately. This is because, although the thickness of the structure is controlled by adjusting the relative speed between the substrate and the nozzle in the film forming apparatus according to the AD method, actually the density of the raw material powder (aerosol density) contained in the aerosol is unstable. In order to solve such a problem, JP-P2001-348659A discloses on the first page that an amount of ceramic fine particles within the aerosol is sensed by a sensor and a signal output from the sensor is fed back to the apparatus for fabricating a ceramic structure so as to generate an aerosol containing many primary particles of ceramic in an amount stable over time and adjust the deposition height of the ceramic structure.
However, it has been known that the aerosol density and the film forming speed are not in proportion strictly. In the AD method, during carriage of the generated aerosol, the micro powder of the raw material (primary particles) is agglomerated by the electrostatic force or the like, and, for example, agglomerated particles (secondary particles) having a diameter of several micrometers or more are formed. Such an agglomeration of particles occurs more easily as the aerosol density is higher. However, such agglomerated particles cannot contribute to the film formation because kinetic energy of the agglomerated particles is consumed for crushing themselves. Accordingly, when the aerosol density is the same, the film forming speed differs depending on the ratio between the primary particles that contribute to film formation and the secondary particles that do not contribute to the film formation contained in the aerosol. Here, the ratio cannot be controlled. Therefore, according to the method of controlling the respective parts of the film forming apparatus on the basis of the consumed amount of raw material powder, the thickness of the structure cannot be controlled accurately, either.
On the other hand, Jun AKEDO et al., “Influence of Carrier Gas Conditions on Electrical and Optical Properties of Pb (Zr, Ti) O3 Thin Film Prepared by Aerosol Deposition Method”, Japanese Journal of Applied Physics, Vol. 40 (2001), pp. 5528–5532, Part. 1, No. 9B, September 2001, The Japan Society of Applied Physics discloses that an electric or optical phenomenon occurs at the time of film formation in the AD method.